Search engines exist to make information accessible. Among the kinds of information promulgated by search engines is advertising. The display of advertisements (“ads”) is often mediated by a bidding system—an advertiser bids on a keyword and the placement of his ad on the search result page for that keyword depends on, possibly among other factors, his bid. Often, the click-through rate on the ad is a function of its placement.
It is in an advertiser's interest to know about as many relevant keywords as possible. Manually generating keywords for a domain is a difficult, labor intensive task—for a given topic there can be many keywords. Additionally, the linguistic behavior associated with search is not entirely like any other, and thus may be difficult to intuit.